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Block

Status: Stable

documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

Description

Block[{x, y, ...}, expr] evaluates expr with local values for x, y, ....

Examples

No verified examples yet for this function.

Implementation notes

Algorithm. builtin_block (in src/modular.c) implements dynamic scoping by save/restore of global symbol state — it does not rename anything (contrast Module). Block carries HoldAll | Protected (set in src/attr.c). For each local x (or x = init) the handler looks up the symbol's SymbolDef, saves its current own_values chain and attributes into a SavedVar record, then clears own_values to NULL so the name is unbound inside the block. If an initializer is given it is evaluated (in the surrounding scope) and installed as a fresh OwnValue on the same global symbol.

The body is then evaluated directly — any reference to x anywhere in the call tree (even inside functions defined elsewhere) sees the block-local value, which is the defining property of dynamic scope. Return[v]/Return[v, Block] is trapped via eval_classify_return. On exit the handler frees the OwnValues created during the block and restores each symbol's saved own_values and attributes, so the global binding is left exactly as it was — even if the body threw or returned early.

Data structures. A SavedVar[] array (name, saved Rule* own-value list, saved attribute bitmask) parallel to the variable list.

  • HoldAll, Protected.
  • Affects only values, not names.
  • Restores original values and attributes after execution.

Attributes: HoldAll, Protected.

Implementation status

Stable — documented, exercised by the test suite and/or worked examples, with no known limitations recorded.

References

Notes & additional examples

Worked examples

In[1]:= x = 10
Out[1]= 10

In[2]:= Block[{x = 3}, x^2]
Out[2]= 9

In[3]:= x
Out[3]= 10

In[1]:= Block[{n = 5}, Sum[k, {k, 1, n}]]
Out[1]= 15

Notes

Block localizes the values of the listed symbols: it temporarily resets them for the duration of the body and restores the outer values on exit. Unlike Module, it does not rename the symbols, so the same global x is reused with a saved-and-restored value.